Generic applications of temperature detectors range from the quantitative measurement of temperature to the generation of an alarm signal when the temperature of an object goes above or below a predetermined level. The latter application area is of great interest to the electronics industry where it is essential to provide fail-safe features in electronic equipment. Such applications are numerous and include power supply systems, computer equipment, television sets, stereo equipment, heating appliances and the like.
Given the wide range of application areas for reliable and precise temperature detectors, product manufacturers often have a supply of several different temperature detectors, such as bi-metallic switches, thermistors, and semiconductor integrated-circuit temperature detectors. Since precision bi-metallic detectors are manufactured to pre-determined temperature transition points with fixed mechanical structures, a product manufacturer must stock different switches for different temperature-detection applications. This, coupled with the expense of bi-metallic switches, adds greatly to the inventory costs for product manufacturers.
Thermistor devices provide a less expensive and more flexible alternative to bi-metallic switches. Thermistors, however, must be configured with additional precision components. Typically, the additional components are precision resistors and a comparator, which are configured into a Wheatstone bridge. Although the additional components reduce inventorying costs by allowing thermistors to be configured for different temperature-transition points, they increase manufacturing costs by increasing component count.
A partial solution to the increased component cost of using thermistors has been provided by semiconductor integrated-circuit (IC) temperature detectors, which integrate some of the above components onto a single chip. These IC temperature detectors generally have four terminals: an input terminal for receiving a stable reference voltage, an output terminal for transmitting a detection signal, a power terminal and a ground terminal. An example of such an IC detector is the LM3911 Temperature Controller manufactured by National Semiconductor Corporation, which will be discussed in greater detail below. However, to provide the stable reference voltage required by such an IC temperature detector, the user must configure precision resistors and additional components around the IC detector.
The above prior art examples are generally directed towards detecting a specific transition point in temperature, such as that which occurs in the over-heating of electrical equipment. Another branch of the prior art related to the present invention is generally directed towards temperature-control systems for house and office environments. Recently, digital electronics have been incorporated into thermostat-control units to allow the user to digitally set the temperature of the house or office environment to within one degree Fahrenheit. Such a digital thermostat-control unit comprises a temperature sensor, a means for generating a plurality of temperature-transition points, a digital data bus comprising several lines for receiving a program code for selecting one of the temperature-transition points, and means for comparing the temperature-sensor signal against the selected temperature-transition point.
Such digital thermostat units provide the flexibility of multiple transition points needed in the area of temperature detectors. They, however, require the generation of program signals for the digital data bus. The generation of such program signals increases the manufacturing costs.
In summary, there is a great need for flexible temperature detector that may be configured, with a minimum of terminals and with the aid of relatively inexpensive and non-precision external components, for a multitude of temperature ranges and applications.